An EKG measurement device typically receives an EKG signal with two EKG electrodes that are connected to inputs of a high-impedance differential amplifier. If there is the option of connecting more than two EKG electrodes to the EKG measurement device, typically one of the EKG electrodes will be used as a common reference point.
An EKG measurement device of the type mentioned above is disclosed in the book by Karsten Meyer-Waarden: “Bioelektrische Signale und ihre Ableitverfahren (Bioelectric signals and their derivation process)”, 1985, Schattauer Verlagsgesellschaft, Stuttgart, Germany, on pages 142 to 143. In the EKG measurement device described therein a noise signal with a common-mode component is reduced at two EKG measurement electrodes with the so-called reference potential control method. The noise signal is created by a displacement current which is produced in its turn here by an electrical power supply network with alternating current. In the reference potential control method the reference electrode is not at reference ground or reference potential, but receives a potential corresponding to the common-mode component of the noise signal. The noise signal is tapped off at the two EKG electrodes. The common-mode component of the noise signal is fed, after amplification, impedance conversion and inversion, to the electrode which determines the reference potential for the measurement. The displacement current coupled into the body thus does not flow against the reference ground at a constant reference potential but flows into a reference point of which the potential is controlled by the noise voltage. The displacement current is compensated for by an opposing current in terms of amount and phase.
EKG devices are used not only for measurement and monitoring of the heart function but also in medical imaging to create trigger signals. Information about the heart phase is obtained from the EKG signal during imaging in order in this way to synchronize the imaging with the heart activity. With imaging processes requiring a longer period to record the image in particular high-quality images of the heart or also images of regions which pulse with the beating of the heart can be created.
EKG measurement devices are therefore also advantageous for in-situ recording of EKG signals during an examination of a patient by means of a magnetic resonance (MR) device. Operation in the magnetic resonance device however demands a series of measures to make trouble-free measurement in the environment of the magnetic resonance device possible at all. It is well known that strong high-frequency fields in the megahertz range as well as strong gradient fields in the low-frequency range are used in the magnetic resonance device for imaging. The EKG measurement may neither be disturbed by the operation of the magnetic resonance device nor may it disturb the operation of the magnetic resonance device itself. EKG measurement devices which are MR-compatible in the sense stated above are available on the market.
However the problem with such devices remains magnetic fields which change over time, as are used in the magnetic resonance device as magnetic gradient fields for location encoding. According to the law of induction, changes to magnetic fields over time create noise voltages which are coupled into the EKG signal received by the EKG electrodes as noise. Movements of the patient during image recording in the static magnetic field also create noise signals in accordance with the law of induction, because the effective surface for the coupling-in is changed by the movement. These types of magnetically-created noise signals overlay themselves with the EKG signal created by the body and falsify this signal.
Recording a magnetic resonance image synchronized with the heartbeat however basically demands a reliable detection of the R wave in the EKG signal. The noise signals generated by the switched gradient fields and also by rapid movements can however be mistakenly interpreted as an R wave and thus, because of the incorrect triggering that they generate, lead to a marked deterioration in the image quality. The practice of investigating the EKG signals in the trigger unit of the EKG measurement device for problems caused by magnetic fields is known. To this end the dynamics of the EKG signals are analyzed and evaluated as to whether the EKG signal involves an R wave to be detected or a fault. Incorrect triggering is still not excluded if the dynamics of the noise signal correspond to those of the R wave in the EKG signal.